usb: gadget: mass_storage: Set max_speed to SSP

Increase max_speed of the mass_storage driver for UDCs that support
SuperSpeed Plus. The composite driver will pass this value to UDC core
to set the device speed on probe (actual speed may be different
depending on whether the USB controller supports it or other external
factors).

Signed-off-by: Thinh Nguyen <thinhn@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>